Since the beginning of agriculture, farmers have developed methods for managing weeds, insect pests and diseases. Due to the significant impact from pests and diseases, on both human and animal health, it is important for those interested in plants to develop a firm understanding of weed science, entomology (study of insects) and plant pathology (study of plant disease), and how to minimize losses caused by these important plant pests.
All plants, native and cultivated, are prone to diseases and injuries. There are many definitions of plant disease. One definition is that disease is suboptimal plant growth brought about by a continuous irritant, such as a pathogen (an organism capable of causing disease) or by chronic exposure to less than ideal growing conditions. In contrast, injury is loss of plant vigor resulting from an instantaneous event, such as a lightning strike, hail damage, chemical burn or mechanical damage. Because of the instantaneous and “cause-and-effect” nature of injuries, they are often easy to diagnose. In the case of diseases, the effects are caused by a continuing process or irritation.
The source of continuous irritation may be abiotic (non-living) or biotic (caused by a pathogen). Abiotic diseases are also referred to as non-infectious diseases as they do not spread from plant to plant. Examples can include nutrient deficiencies growing under too much or too little light, and air pollutants such as automobile exhaust. Biotic diseases are caused by pathogens and are often referred to as infectious diseases, because they can move within and spread between plants. Plant pathogens are very similar to those that cause disease in humans and animals and include viruses, bacteria, fungi, and nematodes.
Pathogens may infect any part of the plant including leaves, shoots, stems, roots, fruit, and seeds. For an infectious disease to develop, a susceptible host, a pathogen capable of causing disease and a favorable environment for the pathogen to grow is required. If any one of these factors is absent, disease will fail to develop. In the case of infectious plant diseases, practices that favor plant growth over pathogen activity tend to decrease the amount of disease observed. For example, plants that are fertilized and watered correctly will be less likely to develop disease.
Regardless of which pathogen, disease development on a plant requires that the pathogen must: (a) come into contact with a susceptible host (referred to as inoculation); (b) gain entrance or penetrate the host through either a wound, a natural opening on plant surface (stomata, lenticels, etc.) or by direct penetration of the host; (c) establish itself within the host; (d) grow and multiply within or on the host; and (e) be able to spread to other susceptible plants. Successful pathogens must also be able to survive long periods of unfavorable environmental conditions in the absence of a susceptible plant host. Together, these steps are referred to as the disease cycle. If this cycle is broken, the disease will be less severe or fail to develop.
Two terms that are often used when discussing plant disease and injury are sign and symptom. In the medical field, the two words are used differently than in the plant world. In dealing with plants, the word sign is used when the pathogen or part of the pathogen is observed in or on an infected plant. Examples include: fungal hyphae or mycelium, spores, fruiting bodies, bacterial cells, or virus particles. Symptoms are visual or noticeable changes of a plant which result from disease or injury. Early after infection, disease symptoms are often invisible since they take time to develop. Examples of common symptoms are: yellowing of leaves; wilting of leaves; dropping of leaves or fruit; and stunting of plant parts or the whole plant.